Synchronous switching system



March 29. 1966 l E, R PERRY ETAL 3,243,610

SYNCHRONOUS SWITCHING SYSTEM Filed Deo. 50, 1963 www in the neighborhood of a current zero. terms zero crossing and current zero will be used to describe this point, the device of this invention can be set to open the breaker at some other point that may be ap- United States Patent O 3,243,610 SYNCHRONOUS SWITCHIN G SYSTEM Elijah R. Perry, Portland, Oreg., and William E. Harper,

Walpole, Mass., assignors, by direct and mesne assignments, to Allis-Chalmers Manufacturing Company,

Milwaukee, Wis.

Filed Dec. 30, 1963, Ser. No. 334,411 2 Claims. (Cl. 307-133) This invention relates generally to electrical circuit interrupters, more specifically to a switch system that has a circuit interrupter, an actuating mechanism for operating the circuit interrupter, and a control circuit for signaling the actuator to separate the circuit interrupter contacts at a current zero in the wave form of the electrical system the circuit interrupter lis connected in.

Circuit interrupters are ordinarily operated atrandom points in the current wave formY of the electrical system. This kind of circuit interruption takes place in two steps. First, the contacts are separated far enough that the contact gap would withstand the system voltage except for vthe presence of the arc that forms between the contacts the contacts, and a substantial part of the cost of a circuit linterrupter is the cost of making it better able to `withstand the arc and to extinguish the arc. terrupters can be made smaller and less costly if the con- Circuit intacts can be opened reliably only at an appropriate point Although the propriate for a particular circuit interrupter or for a parvticular electrical system. However, the problems of providing a suitable switch actuator and control circuit have prevented practical application of current zero switching. An object of this invention is to provide a switch system i that electively separates the contacts of a switch only at a 4,current zero.

One of the problems in interrupting a circuit at a current zero is that after the current zero, the voltage between the contacts rises and may cause dielectric breakdown of the contact gap unless the dielectric strength of the gap increases at a very high rate. The dielectric strength of the gap can be increased fast enough to prevent breakdown by separating the contacts at a high rate. A related problem is the need to keep the elapsed time between contact separation and the occurrence of the current zero to the absolute minimum which will assure that the current zero occurs immediately after contact separation. The longer the arc persists and the larger the value of arc current, the greater will be the resulting contamination of the contact gap. In' other words, if the contact separation is poorly timed, the problem of separating the contacts fast enough to avoid dielectric breakdown on the rising edge of the voltage wave form is aggravated.

A further problem in achieving current Zero contact separation is caused by the fact that all known mehanical switches operate very slowly in comparison with the half period of a sixty cycle per second sinusoid of an electrical i 3,243 ,6 l()| Patented Mar. 291, 1966 system. Unless the switch is signaled sufficiently ahead of time so that with the delay in operating the switch, the contacts are separated at a current zero. However, since the wave form may change, such circuits are reliable only for short intervals.

In all the problems that have been discussed, the speed of separating the contacts is a serious limitation. With slow separating contacts the rising slope of the voltage wave form is more liable to break down the dielectric and produce an arc.

Slow operating switches require predicting the shape of the wave form rather far ahead and thereby increase the chance that the actual wave form will deviate from the assumed wave form and cross zero sooner or later than the predicted time. A more speciiic object of this invention is to provide a switch and a switch actuator that are fast opening in response to a trip signal.

Current zero interruption has also been proposed for D.C. circuits. These circuits are provided with a resonant inductance-capitance circuit that superimposes a damped sinsuoid on the D.C. system. The switch and the resonant circuit are operated together so that the contacts separate at a peak in the sinusoid where the total line current is zero. Known prior art switches have been slow operating and have required impracticably large inductors and capacitor banks to produce a sinusoid of a sufliciently long period to match the slow switches. Another object of this invention is to provide a new and improved switch system that can be used in direct current systems with smaller resonant circuit components.

The system of this invention includes a switch that has especially low mass moving parts and requires only a short separation between the contacts; commercially available vacuum switches meet this requirement. The switch system includes a mechanical actuator for the switch that has low mass stored energy components for driving the contacts open. It also includes a control circuit that produces a trip signal appropriately ahead of the predicted line current zero for the switch to operate properly.

In the drawing the single figure is a schematic of the switch Asystem of this invention.

Introduction-The switch system will be described as it is used in a single phase alternating current system 10 that includes a generator 11 that supplies a load of lumped and distributed parameters represented by a resistor 12, an inductor 13, and a capacitor 14 connected in lines 15 and 16. A switch 20 is connected in system 10 to control the part of the system shown in the drawing.

The switch system has a current zero predicting circuit 21 that is coupled to system 10 to receive a measure of the current wave form. Current zero predictor 21 operates on the measure of the wave form to produce a signal that is properly timed to start switch 20 opening appropriately ahead of a zero crossing so that it will have its contacts effectively separated at the zero crossing. An amplilier stage 23 receives the zero predicting signal and properly amplies it to operate an actuator 25 that physically separates the contacts of switch 20. The control also has an initiating stage 26 that inhibits the transmission of information or orders through the control except when it is intended to open switch 20. As the drawing illustrates the invention, initiating stage 26 controls the transmission of the current zero predicting signal from current Zero predicting circuit 21 to amplifier 23.

All of these components have electrical and mechanical reactances that cause a predetermined time delay between a current zero predicting signal and the resulting separation of the contacts of switch 20.

The switch-Switch 20 is constructed to have a low mass in its moving parts and to establish a high dielectric strength medium between its contacts with only a small separation between them. Commercially available vacuum switches meet these requirements.

Current zero predicto1x-The current zero predictor 21 is coupled to line 1,6 (or line by means of a current transformer 30. Current transformer 30. may have an air core to provide a suitable phase relationship between the primary and secondary currents. The primary winding 30p of current transformer 30 may comprise a straightthrough section of line 16; its secondary winding 30s is connected to produce the line current wave form in the primary winding of `one or more peaking transformers 31 and 32. A peaking transformer is a saturable core device that is operated in both its saturated and its unsaturated regions; it is constructed to have a sharpI knee hysteresis characteristic so that the transition between these regions takes place at a well defined point in the wave form of the current component exciting thevcore. While the core is saturated, substantially no voltage appears across the windings, and a series of alternating polarity pulses appear across the windings as the core passes through its unsaturated region. Peaking transformers 31, 32 have their windings series connected to provide a suitably shaped voltage pulse. -By well known phase shift techniques, the peaking transformers are arranged to produce their pulses ahead of the zero crossing in the. line current wave form by the predetermined delay in operating switch (for the expected current amplitude).

The switch action initiation-The switch initiator 26 performs two functions; it generates a two condition signal indicating whether switch 20 is to remain closed or to be opened, and it responds to this signal by transmitting the current zero predicting pulses from stage 21 to stage 23. The signal generating function may be performed by well known fault detecting devices that are not a part of this invention and by the manual control that will be described in detail. i l

As the drawing illustrates initiator 26, it comprises a switch 34v havng a movable contact 35 that is counectable to either of two fixed contacts 36 or 37. Fixed contact 36 connects movable contact 3S. to one end of the secondary windings of peaking transformers 31, 32 through a resistor 38, and fixed contact 37 connects movable contact 35 directly to the other end of the peaking transformer secondary windings. Ay thyratron 39 is connected to conduct across the 'secondary terminals as an uncontrolled diode when contact 35' is in the hold closed position of fixed contact 36 and to presentan open circuit to the windings when movable contact 35 is in the open position of fixed contact 37. Thyratron 39 has its anode 39a connected to contact 36 through resistor 38, its grid 39g connected to movable contact 35, and its cathode 39e connected to fixed contact 37. Resistor 38 has the appropriate resistance to limit the grid current of thyratron'39. Thyratron 39 presents a desirably low burden to current transformer 30 during pulses of the polarity to turn on the thyratron.

As the connections between current zero predictor 21 and initiating control 26 have been described so far, the control would produce a switch actuating pulse once in each cycle of the line current wave form. To operate on either half cycle, components of initiating control 26 and amplifier 23 may be duplicated to respond to pulses of both polarities or the pulses can be full wave rectified to operate thyratron 39 on each half cycle. In some electrical systems it is desirable to open a switch only on a half -cycle of a preselected polarity as the control of the drawing provides.

The amplifier stage-Amplifier 23 responds to the presence of a pulse of appropriate polarity across the power terminals 39a, 39e of thyratron 39 to produce an output of suicient amplitude and duration to start actuator 25 operating. Preferably power stage 23 operates with a low impedance source of stored energy that is independent of system 10 during a switching operation; with the stored energy source amplifier 23 transmits the zero current predicting pulse with an appropriately fast rise time. As the drawing illustrates the invention, power stage 23 comprises a battery 43, a capacitor 44, a resistor 45 connecting capacitor 44 to be charged by battery 43, a thyratron 46 having its cathode and anode terminals connected to switch capacitor 44 to produce a high current pulse at the output terminals 47, 4 3 of stage 25, and a capacitor 49 connected across the output terminals. Capacitor 44 has an appropriate capacitance to provide all the energy required to signal actuator stage 25; resistor 45 has sufficient conductance to charge capacitor 4,4 to approximately the voltage of battery 53 within the minimum time between operations of switch 20. Thyratron 46 has its grid and anode terminals connected by an array of resistors 50, l51 to receive the zero predicting pulses from stage 21. Capacitor 49 has an appropriate capacitance to somewhat widen the pulse at the output terminals. i

The actuaron-Actuator 25 includes a source 515 of stored energy and a mechanism 56 that is responsive to the output of amplifier 23 to release the energy to operate a motor 57 that physically separates the contacts of switch 20. Motor 57 is preferably an extensible and contractible fluid motor having a cylinder 59 and a piston 60. Source 55 comprises an accumulator 62 and a pump 63 and reservoir 64 that are connected to charge the accumulator after switch 20 is operated. Mechanism 56 is illustrated as a valve 66 connecting accumulator 62 to side 67 of cylinder 59 and a torque motor 68 connected to be energized by the amplifier 23 and to turn valve 66 open.

When the movable contact 35 of switch initiator 2.6 is placed in the open position of fixed contact 37, thyratron 39 is kept nonconducting and the next zero crossing of the preselected polarity in system 10 is preceded by a zero predicting pulse across the windings of peaking transformers 31, 32. In response to the zero predicting pulse, thyratron 46 turns on and connects capacitor 44 to drive motor 68 to open valve 6,6. Opening valve 66 permits the fluid pressure to operate motor 57 to open the contacts of switch 20.

Switch 21,0 is reclosiedl and held closed by any suitable means, preferably by a suitable connection of the pressurized fluid system to side 69 of cylinder 5-9. The drawing shows the part of this connection that is operable during opening, a connection between cylinder 59 and reservoir 64.

Those skilled in the art will recognize variations in components and different equivalent functional relationship within the spirit of the invention and the scope o f the claims.

Having now particularly described and ascertained the nature of our said invention and the manner in which it is to be performed, we declare that what we claim is:

'1. A circuit interruptersystem for operation at current zeros in an electrical system, comprising,

a circuit interrupter operable between a first condition and a second condition,

a source of stored energy, including a charge of pressurized fluid for operating said circuit interrup-ter to switch from one condition to the other,

a control circuit adapted to be coupled to said system to receive a measure of the current wave form of said system and operable to produce a signal timed with respect to the delay in operating the circuit interrupter to control said interrupter to reach its second condition at a current zero in said system,

5 6 means cooperating with `said source of stored energy References Cited by the Examiner rrliccrpglnsigrdto said signal to operate said circuit UNITED STATES PATENTS means for initiating a switching operation. 2,564,179 8/1951 TejadflFlores w 317-11-1- 2. A circuit interrupter system according to claim 1 5 218251824 3/1958 Perohm 307-133 in which said means responsive to said signal comprises 2878A02 3/1959 Kfhn 307-133 a fluid motor connected to operate said circuit interrupter, 2,988,672 6/1961 Kll'tz 307-133 a valve operable to release said pressurized uid to operate said uid motor, and an electric motor connected to LLOYD MCCOLLU'M Prlmary Examiner' operate said valve in response to said pulse. 10 W. M. SHOP, Assistant Examiner. 

1. A CIRCUIT INTERRUPTER SYSTEM FOR OPERATION AT CURRENT ZEROS IN AN ELECTRICAL SYSTEM, COMPRISING, A CIRCUIT INTERRUPTER OPERABLE BETWEEN A FIRST CONDITION AND A SECOND CONDITION, A SOURCE OF STORED ENERGY, INCLUDING A CHARGE OF PRESSURIZED FLUID FOR OPERATING SAID CIRCUIT INTERRUPTER TO SWITCH FROM ONE CONDITION TO THE OTHER, A CONTROL CIRCUIT ADAPTED TO BE COUPLED TO SAID SYSTEM TO RECEIVE A MEASURE OF THE CURRENT WAVE FORM OF SAID SYSTEM AND OPERABLE TO PRODUCE A SIGNAL TIMED WITH RESPECT TO THE DELAY IN OPERATING THE CIRCUIT INTERRUPTER TO CONTROL SAID INTERRUPTER TO REACH ITS SECOND CONDITION AT A CURRENT ZERO IN SAID SYSTEM, MEANS COOPERATING WITH SAID SOURCE OF STORED ENERGY AND RESPONSIVE TO SAID SIGNAL TO OPERATE SAID CIRCUIT INTERRUPTER, AND MEANS FOR INITIATING A SWITCHING OPERATION. 